This invention relates to wireless receiver, and in particular to channel estimation in a receiver for a wireless network such as a wireless local area network (WLAN).
Wireless transmissions such as wireless data transmissions in a wireless network are distorted in transit by the radio channel, i.e., distorted versions of the transmitted signals are typically received at a wireless receiver. Equalization is a term that refers to the measures taken to reduce the distortion effects in a radio channel, and this typically requires estimating the channel or its inverse.
Orthogonal Frequency Division Multiplexing (OFDM) is increasingly being used in wireless networks. The IEEE 802.11 WLAN standard, for example, includes the 802.11a and the 802.11g variants that use OFDM. In a wireless receiving station that uses OFDM, channel estimation is used to determine the relative channel responses of the different subcarriers and to correct for variation in such channels. Channel estimation is also used to provide information on the quality of the channels that can be used in demodulating and decoding-such information commonly referred to as channel state information or CSI.
The OFDM variants of the IEEE 802.11 standard use data packets that each include a preamble that provides for channel estimation in a receiver. In particular, two known “long” symbols are used. Prior art channel estimation techniques include averaging the two estimates of the channel obtained from the two long symbols. Noise and other factors reduce the quality of this channel estimate, and such quality reduction lowers the quality of subsequent communication, e.g., the bit error rate (BER) and/or the packet error rate (PER).
Thus there is a need for improved estimation. Furthermore, the channel may change during the time that a packet is being received, i.e., within a packet.
Thus there is a further need for improved channel estimation by tracking changes during a packet.
The research literature includes many papers on channel estimation and channel tracking. See for example, H. Schmidt, V. Kühn, K. D. Kammeyer, R. Rückriem, and S. Fechtel: “Channel Tracking in Wireless OFDM Systems,” CSI 2001, Orlando, Fla., USA, 22-25 Jul. 2001. The techniques described are typically computationally complex, e.g., involving optimization computations.
Thus, there is still a need for a practical relatively simple method that works well, yet that lends itself to inexpensive low power implementation in a chip.
Above-referenced and incorporated-by-reference U.S. patent application Ser. No. 10/217,117 (the Parent patent application now U.S. Pat. No. 7,161,896) describes a practical method that improves the quality of the channel estimates using the possibility that there may be correlation between the channels of the subcarriers.
However, there is still a need to provide for tracking of channel estimates. This is preferably carried out by a simple implementation that has low computational complexity and lends, itself to simple low power implementation.